Abstract
Changes in intracellular or extracellular solute concentration generate a transmembrane osmotic gradient. Animal cell membranes being non-rigid and generally freely permeable to water, any such gradient results in flow of water into or out of the cell inducing cell volume perturbation. Regulation of cellular volume, which is important for cells to function properly, is achieved by activation of membrane transport processes. In response to swelling, cells that undergo a regulatory volume decrease (RVD) adjust their volume through the release of intracellular osmotically active solutes (termed “osmolytes”) accompanied by an obligate water loss. “Inorganic osmolytes” (mainly K+ and Cl-) play a central role in the RVD response, leaving cells via a K+-Cl- cotransport system (see Chap. 9) and/or volume-activated channels (Sarkadi and Parker 1991). However, most cells also contain at high concentrations (tens to hundreds of millimolar) small organic, osmotically active solutes which exert a stabilizing influence on intracellular proteins and are synthesized from metabolic precursors or taken up from the extracellular medium via energy-dependent transport mechanisms. These “organic osmolytes” fall into 3 different classes: amino acids (e.g. taurine), polyols (e.g. sorbitol and myo-inositol) and methylamines (e.g. betaine). For a wide range of cells, organic osmolytes are also released in response to cell swelling (Strange and Jackson 1995; Kirk 1997) and they may account for as much as half of the total RVD (Garcia-Romeu et al. 1991).
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